1. Field of the Invention
The present invention relates to a silicon nitride sintered member having an excellent heat insulating property. More specifically, the invention relates to a silicon nitride sintered member having the surface coated with zirconia and a low thermal conductivity near the surface.
2. Related Art Statement
Since silicon nitride sintered members are more excellent than other inorganic materials or metals in terms of mechanical strength, thermal resistance, thermal shock resistance, corrosion resistance, etc. at high temperatures, its uses as high temperature structural members have been examined, and the uses has actively been being developed.
However, the silicon nitride sintered members have a shortcoming that it has a high thermal conductivity and therefore it is poor in heat insulating property. For instance, when a silicon nitride sintered member is used at a wall face of an engine combustion chamber to enhance the thermal resistance, heat insulation to the exterior can be improved by an air gap, etc, but proper intake gas density can not be obtained because the thermal conductivity is high and therefore intake gas is warmed. In that case, the high thermal conductivity near the surface of the silicon nitride sintered member comes into a problem.
It is disclosed in Japanese patent application laid-open Nos. sho 55-28,351, 57-140,876 and 58-87,273 that the surface of a metal substrate is coated with zirconia through plasma spraying, aiming at the improvement of thermal resistance, corrosion resistance and wear resistance. Further, Japanese patent application laid-open Nos. sho 57-135,771 and 58-125,679 that the surface of a substrate of carbon is coated with zirconia. In such a technique, a cermet, platinum or Ni base alloy is used as an intermediate layer for strengthening a bonding force of the zirconia layer. However, there has been no idea that a silicon nitride sintered body is used as a substrate and the thermal insulation near the surface thereof is intended to be improved.
As compared with the silicon nitride sintered body, zirconia has a lower thermal conductivity and a more excellent thermal insulation property, but it has a defect that it is lower in mechanical strength at high temperatures.